1. Field of the Invention:
The present invention relates to a semiconductor memory device having a redundant structure and, more particularly to a semiconductor memory device having means for indicating the state of the redundant structure of the device.
2. Description of the Related Art:
In a semiconductor memory device, a plurality of memory cells are arranged in a matrix form of rows and columns. The possibility of defects generated in a semiconductor memory device during the manufacture thereof has been increased in accordance with the increase in memory capacity of the memory device. Therefore, the higher the memory capacity or integration density of the device, the greater the number of the defective memory cells. However, even if the device includes only one defective memory cell, the device cannot be operated normally and, the device must be abandoned.
In order to save such a memory device having a small number of defective memory cells, a redundant structure has been employed in which a defective memory cell or cells in a normal memory cell array are functionally replaced by a redundant memory cell or cells of a redundant memory cell array. Thus, the manufacturing yields can be improved in the semiconductor memory device by the redundant technique.
In such a semiconductor memory device incorporating the redundant structure, from the point of view of quality control, it is important to know whether or not the redundant structure such as a redundant memory cell array is actually being used. It is also important to know the address of the normal memory cell array replaced by the redundant memory cell array in view of defect analysis and function test. On approach for indicating the status of the redundant cell array is to provide a so-called roll-call circuit in the memory device. The conventional roll-call circuit includes a programmable element such as a polysilicon fuse and a current sink circuit connected to a predetermined input terminal. When the redundant cell array is used to replace the defective memory cell or cells of the normal memory cell array, the programmable element is programmed to a predetermined state to enable the current sink circuit. The enabled current sink circuit causes a leakage current flowing from the predetermined input terminal when a voltage outside the normal voltage range is applied to the predetermined input terminal. Thus, the state of the redundant cell array can be detected by the leakage current flowing into the predetermined input terminal.
The semiconductor memory having the aforementioned roll-call circuit of the prior art also examines the address using the redundant row or column in terms of the input leakage current measured. However, it takes 10 msecs or more for the existing tester to measure that input leakage current. In case the semiconductor memory has 512 rows and one redundant row, for example, it takes about 5 secs to locate the address of the reundancy row.
On the other hand, the functional test before shipping has to be accomplished under the most severe conditions, which are changed because the relations between the physical arrangement of the cell information and the address are changed by using the redundant cell array.
During the function test, therefore the address using the redundant cell array has to be examined, and the cost for the semiconductor memory is raised by the aforementioned long time taken for the test.